Conventional optical security devices provide authentication by manipulating a specific property of light to produce a distinctive optical signature. Microscopic color prints modulate the amplitude and holograms modulate the phase of light, but the researchers believe that this structure can be easily imitated. So they designed a pixel that overlays a structural color element onto a phase plate to control both the phase and amplitude of light. They then arrayed the pixels into monolithic prints, with each pixel strategically arranged on a plane. Nanostructured posts with different heights were used as structural colored filters to modulate the amplitude of light.

The researchers also created an algorithm that takes multiple images as its input and generates an output file to determine the positions of different phase and colored filter elements. They then used a nanoscale 3D printer to create a holoscopic print of painter Luigi Russolo’s 1910 painting “Perfume.” The color print is visible under ambient white light. Different thicknesses of polymerized cuboid were used to modulate the phase plates and form three multiplexed holograms, projected as a red thumbprint, a green key, and blue letters that read “SECURITY.” The images were embedded within the print.

The holographic color prints can be easily verified, according to the researchers, but are difficult to imitate.

“The relationship of holograms in combating counterfeiting is analogous to antibiotics against infections,” said professor Joel Yang. “Every so often, new technology is needed to deter counterfeiters as the old-fashioned holograms become easier to copy.”

The prints consist of nano 3D printed polymer structures and can be used in optical document security. Information in the prints is encoded only in the surface relief of a single polymeric material, so nanoscale 3D printing could then be used to mass manufacture customized masters by nanoimprint lithography.

“For the first time, multiple holograms that are color selective are ‘woven’ into a colorful image using advanced nanofabrication techniques,” Yang said. “We are hopeful that these new holographic color prints are user friendly but counterfeiter unfriendly: They are readily verified but challenging to copy, and can provide enhanced security in anticounterfeiting applications.”